1. Field of the Invention
The present invention relates to Joule-Thomson cryostats. More specifically, the present invention relates to systems and techniques for improving the performance of Joule-Thomson cryostats.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
A cryostat is an apparatus which provides a localized low-temperature environment in which operations or measurements may be carried out under controlled temperature conditions. Cryostats are used to provide cooling of infrared detectors in guided missiles, for example, where detectors and associated electronic components are often crowded into a small containment package. Cryostats are also used in superconductor systems where controlled very low temperatures are required for superconductive activity.
A Joule-Thomson cryostat is a cooling device that uses a valve (known in the art as a "Joule-Thomson valve") through which a high pressure cryogen is allowed to expand via an irreversible throttling process in which enthalpy is conserved, resulting in lowering of its temperature.
The simplest form of a conventional Joule-Thomson cryostat typically has a fixed-size orifice in the heat exchanger at the cold end of the cryostat such that cooling by the cryostat is unregulated. The input pressure and internal cryogen flow dynamics establish the flow parameters of the cryogen through the cryostat. Unfortunately, as is well known in the art, rapid cool-down requires high rate gas flow and a large size orifice, while long cooling durations require demand flow, where flow refrigeration capacity just offsets heat load. With a passive fixed orifice cryostat, demand flow operation is not inherent and simultaneous rapid cooldown and long cooling duration are mutually exclusive. These two conditions cannot be simultaneously met in a fixed orifice cryostat. Accordingly, although the conventional Joule-Thomson cryostat is a simple apparatus in that it has no moving parts, the inherent, uncontrolled flow characteristics make the fixed-orifice type cryostat unsuitable for many applications where rapid cool-down and long cooling durations with limited cryogen are required.
Since approximately the 1950's, demand-flow Joule-Thomson cryostats with internal, passive, thermostatic control of variable orifice size have been used. These cryostats have fluidic throttling valves which provide the ability to start cool-down with the maximum orifice size, thereby providing high rate cryogen flow and refrigeration for rapid cool-down. After cool-down is achieved, the orifice size is reduced by the valve for minimal cryogen flow rate and sustained cooling for the thermal load. The fluidic throttling valve generally includes a passive thermostatic actuator within the mandrel of the apparatus which provides self-regulation of cryogen flow based upon the temperature in and around the cryogen plenum chamber. The cooling rate is proportional to the mass flow rate of cryogen through the cryostat. The thermostatic element is conventionally a fluid-filled bellows or a segment of specifically selected monolithic actuator material which contracts or expands as the temperature changes. As the actuator changes temperature, it changes length either due to a phase change in the bellow's charge fluid, or it changes length due to the material's expansion properties.
The thermostatic actuator is coupled to a demand-flow needle valve mechanism. As the temperature drops, the actuator is adapted to contract and cause the needle to extend into and partially close the Joule-Thomson orifice. At the predetermined critical temperature, the thermostatic element closes the needle valve entirely. As the temperature rises, the element expands again and actuates the valve mechanism, allowing new cryogen flow through the orifice and ultimately to the heat load.
Many applications, such as missile applications, require low sustaining flow rates to achieve long required run times. The allocated mass and volume of either a pressure vessel source or a compressor source for this type of application is aggressively minimized because of vehicle constraints, thus cryogen supply is limited. These applications spurred the development of cryogen efficient demand flow cryostats, where flow refrigeration capacity equals heat load over all environmental conditions. As a result, during low heat load conditions such as low environmental temperatures, cryogen flow diminishes to very low values and the needle valve becomes almost closed.
Coincident with sustaining operation at low flow conditions, a thermostatic actuator requires relatively large travel to fully open the needle valve and achieve high initial flow rates for quick cooldown. Cryostats that perform best for these conditions incorporate bellows type thermostatic actuators requiring exacting fabrication, charge fluid parameters and initial valve adjustment.
A technique to provide a minimum flow rate is to incorporate a flow bypass within the needle valve. This typically consists of a slight channel in either the needle valve's needle or orifice which leaks cryogen when the valve is fully seated. Although this feature prevents extremely low flow, flow is set low to prevent excessive cryogen consumption. This feature is also difficult to fabricate consistently.
The critical problem with this type of cryostat and all cryostats exhibiting low flow rates is a lack of reliability. For many reasons, flow can be interrupted and the controlled temperature increases unacceptably before the actuator responds and flow is resumed. The most persistent problem is cryogen contamination. Impurities in the cryogen precipitate out which blocks the orifice and/or seizes the needle, inhibiting valve operation and the flow of cryogen. Additionally, the cryogen itself can precipitate out upstream of the orifice and subsequently disrupt needle valve operation. This specific phenomena is most prevalent right after cooldown when excess coolant is produced and is mostly associated with Argon operation. The response time of the thermostatic actuator is too long to prevent flow interruption of sufficient duration to cause unacceptable temperature rise.
Further, bellows type cryostats must maintain a very good seal over design life and bellows are difficult to manufacture to the specifications a cryostat application requires. Designs that utilize a material's coefficient of thermal expansion (CTE) are simpler than bellows design, but have the disadvantages of low sensitivity and slow thermal response. This limits the reliability of such designs at low flow rates and at all but the highest purities of cryogens.
In addition to bellows type cryostats, cryostats which utilize a monolithic high CTE elastomeric material have been developed. See U.S. Pat. No. 4,152,903, issued May 8, 1979 to Ralph C. Longsworth and entitled Bimaterial Demand Flow Cryostat, the teachings of which are incorporated herein by reference. This design attains rapid cooldown and low sustaining flow rates, but proved to be less than fully reliable at low flow rates. Its performance is sensitive to the quality of the cryogen to the point where it is impractical for most applications. And, although this type of actuator can be carefully adjusted for one type of cryogen, it is not amenable for multiple cryogen types.
Another attempt to attain rapid cooldown and reliable long run times was to incorporate a semi-active actuator. This approach is described and claimed in U.S. patent application Ser. No. 08/469,163, filed Jun. 6, 1995 by Matthew Skertic et al., and entitled Adaptive Orifice Joule-Thomson Cryostat With Servo Control (PD 92396), the teachings of which are incorporated herein by reference. The actuator is a wire whose material changes phase near the cryostat operating temperature. With the change in phase, the material significantly changes length. The change in length operates the needle valve in the same manner as a bellows operates the needle valve. When current is passed through the wire, the wire heats and changes temperature. By externally controlling the application of current, heating of the wire is controlled and needle opening or closing is thereby controlled. The difficulties with this approach are that the wire presents structural problems. Maintaining tension in the wire and making the design work in dynamic environments are stressing requirements.
Thus, a need exists in the art for a responsive thermo-active element for a Joule-Thomson cryostat which affords rapid cooldown, reliable demand (low) sustaining flow rates, and operation with multiple cryogen types. In addition, there is a need for a selfcleaning cryostat offering robust performance handling a range of cryogen impurities at low cryogen flow rates.